1. Field of the Invention
The present invention relates to a wireless imaging system which performs imaging control operations such as an exposure start operation and an exposure end operation by wireless communication, a wireless communication apparatus included in the wireless imaging system, a method of controlling the wireless imaging system, and a storage medium.
2. Description of the Related Art
Recently, in the industrial equipment field, there has been developed an advanced device which selects and holds one of randomly stacked components or automatically detects an abnormality by processing an image captured by a camera.
A camera is sometimes attached to the distal end of a robot arm to perform imaging near a processing target. However, there is a risk that a cable will deteriorate and break by repeated deformation. Therefore, there are increasing needs for a technique of performing this operation by wireless communication.
Camera Link is a standard for cabling and interfacing for imaging control and image acquisition with respect to industrial cameras. In this case, imaging control means consecutive control operations required for imaging, for example, an exposure start operation, the light emission start operation of an electronic flash device, a light emission end operation, and an exposure end operation. On the other hand, Japanese Patent Laid-Open No. 2008-102337 discloses a technique for controlling an external electronic flash device from a camera by wireless communication.
When transmitting imaging control information by wireless communication, there are three challenges to be overcome.
The first challenge is to maintain the time intervals between the respective control operations. In general, when performing wireless communication, the delay time in communication changes each time because of MAC (Media Access Control) processing for avoiding wireless communication collision with other peripheral devices and retransmission control for the occurrence of a communication failure. On the other hand, imaging control requires very high accuracy concerning the time intervals between the respective control operations. If, for example, the time from the start of exposure to the end of exposure or the time from the start of light emission of imaging illumination to the end of light emission changes even slightly, overexposure or underexposure will occur. Likewise, a camera using a rolling shutter requires high accuracy concerning the time interval from the start of exposure to the start of light emission.
The second challenge is to reduce a delay time. For example, a robot designed to hold a component uses a captured image for feedback control. In such industrial equipment, if the start of imaging is delayed, the acquisition of an image is delayed. This increases the cycle time for operations. For this reason, for example, it is sometimes impossible to make the wireless transmission side transmit imaging control information altogether after the end of the imaging control upon recording the information together with time stamps and to make the wireless reception side reproduce the imaging control from the start.
The third challenge is to effectively use a wireless band. The technique disclosed in patent literature 1 is designed to secure a band by performing MAC processing in advance before the start of imaging and then occupy the band by the required time. This method can suppress fluctuations in delay time caused by MAC processing. However, if a plurality of industrial devices are placed near to each other, the time during which a peripheral device cannot perform wireless communication prolongs, resulting in an increase in cycle time.
The present invention provides a wireless communication technique of, when transmitting a plurality of pieces of consecutive control information by wireless communication with varying delay times, maintaining the time intervals between the respective control operations with a minimum delay and occupied band.